As a sustainable energy resource, the use of solar cell modules is rapidly expanding. One preferred way of manufacturing a solar cell module involves forming a pre-lamination assembly comprising at least 5 structural layers. The solar cell pre-lamination assemblies are constructed in the following order starting from the top, or incident layer (that is, the layer first contacted by light) and continuing to the backing layer (the layer furthest removed from the incident layer): (1) incident layer (typically a glass plate or a thin polymeric film (such as a fluoropolymer or polyester film), but could conceivably be any material that is transparent to sunlight), (2) front encapsulant layer, (3) solar cell component, (4) back encapsulant layer, and (5) backing layer.
The encapsulant layers are designed to encapsulate and protect the fragile solar cell component. Generally, a solar cell pre-lamination assembly incorporates at least two encapsulant layers sandwiched around the solar cell component. Suitable polymeric materials used in the solar cell encapsulant layers would typically possess a combination of characteristics such as high transparency, low haze, high impact resistance, high penetration resistance, good ultraviolet (UV) light resistance, good long term thermal stability, adequate adhesion strength to glass and other rigid polymeric sheets, high moisture resistance, and good long term weatherability. In addition, the optical properties of the front encapsulant layer may be such that light can be effectively transmitted to the solar cell component.
The use of ionomers, which are derived from partially or fully neutralized acid copolymers of α-olefins and α,β-ethylenically unsaturated carboxylic acids, in solar cell encapsulant layers has been disclosed in, e.g., U.S. Pat. Nos. 5,476,553; 5,478,402; 5,733,382; 5,762,720; 5,986,203; 6,114,046; 6,187,448; and 6,660,930, U.S. Pat Appl. Nos. 2003/0000568; 2005/0279401; 2006/0084763; and 2006/0165929, and Japanese Pat Nos. JP 2000186114 and JP 2006032308. Similar ionomer compositions have also been used as safety glass interlayers (see, e.g., U.S. Pat. Nos. 3,344,014; 3,762,988; 4,663,228; 4,668,574; 4,799,346; 5,759,698; 5,763,062; 5,895,721; 6,150,028; 6,432,522, U.S. Pat Appl Nos. 2002/0155302 and 2002/0155302, and PCT Appl Nos. WO 99/58334 and WO 2006/057771).
Terionomers, which are derived from partially or fully neutralized acid terpolymers of α-olefins, α,β-ethylenically unsaturated carboxylic acids, and α,β-ethylenically unsaturated carboxylic acid esters, have also been used in forming solar cell encapsulant layers (see e.g., U.S. Pat Appl No. 2006/0165929 and Japanese Pat No. JP 2006032308). Similar terionomer compositions have also been used as safety glass interlayers (see; U.S. Pat. Nos. 3,344,014 and 5,759,698, and U.S. Pat Appl No. 2007/0154694).
However, the solar cell encapsulant layers formed of such ionomers often fail to provide adequate protection to the solar cell component from shock. Additionally, the low light transmission and low adhesion to other laminate layers, especially after severe environmental aging, also hinders the use of such ionomers in solar cell encapsulant layers. On the other hand, in addition to low light transmission and low adhesion to other laminate layers, less heat resistance is associated with solar cell encapsulant layers derived from the terionomers, due to their low melting points.
There is a need for polymeric films or sheets suitable as solar cell encapsulant layers, which are transparent, highly adhesive to other laminate layers, and moisture and heat resistant.